Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Mon Dec 4 2023 Valid 12Z Thu Dec 7 2023 - 12Z Mon Dec 11 2023 ...Pattern Overview... The ridge axis that will be in place across the north-central U.S. mid-week is expected to flatten out by Thursday while a ridge then builds across the eastern U.S. to close out the week. Pacific shortwave energy reaching the West Coast Thursday night will help build an upper trough across the western U.S. Friday and into Saturday, and this will then eject eastward across the Plains and spur surface cyclogenesis over the weekend across the central Plains. This will likely result in a well organized storm system tracking northeast towards the Ohio Valley and Great Lakes, with increasing rain and storm chances ahead of the cold front, and wintry precipitation northwest of the surface low. ...Guidance Evaluation/Predictability Assessment... The 00Z model guidance suite is in good overall agreement for the end of the week, and a general model compromise works well as a starting point in the forecast process. There were some differences with the CMC regarding the northwestern placement of the surface low crossing near the U.S./Canadian border, so that model was weighted slightly less than the better clustered GFS/ECMWF/UKMET. In terms of the developing central-eastern U.S. storm system, the guidance has come into better agreement on the timing of the low and the trailing cold front, in contrast to yesterday when there were still major run-to-run variations. The ECMWF has trended more to the west with the 00Z run with the main low, and a triple point low closer to where the GFS/CMC have their main lows. In any case, there will likely be a sub-1000mb surface low tracking southwest to northeast across the eastern U.S. on Sunday and even stronger into Monday. Ensemble means accounted for up to 50% of the forecast by next Monday. ...Weather/Hazards Highlights... The next round of rainfall for the Pacific Northwest is expected to be considerably lighter on Thursday compared to the ongoing atmospheric river event in the short range forecast period, and although some portions of the Oregon coastal ranges may get an additional inch of rain, it does not appear the intensity nor the overall totals are currently high enough to merit any excessive rainfall concerns, although local rivers may still be running high. Even less precipitation is expected here on Friday, before the next round of Pacific moisture advects inland next weekend. The developing western U.S. trough by the end of the week is expected to result in moisture advecting inland across the Intermountain West and the Rockies, with widespread moderate to locally heavy snow for the higher elevations. There may also be some snow for adjacent portions of the western High Plains as the low exits the Rockies. Farther south in the warm sector of the developing surface low, increasing coverage of showers and thunderstorms can be expected from the central Gulf Coast to the Ohio Valley, along with breezy to windy conditions. This storm system should then reach the East Coast by late Sunday into Monday with widespread rain from the Mid-Atlantic to New England, with some snow behind the system across the central and northern Appalachians. Pleasantly mild conditions for early December are expected from the central Plains to the Dakotas and Montana on Thursday, with temperatures running 15 to 30 degrees above average in many cases. The mild conditions then reach the Midwest by Friday and much of the eastern U.S. for the weekend with widespread 10-20 degree above average temperatures likely, with gradually cooling temperatures across the Plains. Colder weather envelops much of the interior West and Rockies by the weekend in response to the upper trough/low. Hamrick Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw